The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
In the prior art, watercraft, such as seaplanes and their sub category flying boats and floatplanes, are known with a wide variety of hull configurations. A seaplane is a general category for aircraft that can operate on water. Flying boats (also sometimes confusingly called seaplanes) have a dedicated hull while floatplanes have external floats, sometimes detachable. To operate on water, these seaplanes require hulls that operate at low, intermediate, and high speed while providing static and hydrodynamic buoyancy and lateral and longitudinal stability while varying speed and attitude (pitch or trim) from rest to takeoff and from first water contact while landing to stationary.
The exact shape of the hull often differs from one waterborne aircraft to the next, from single to multiple.
Higher speed waterborne aircraft operate across a wide range of different speeds. Accordingly, it is possible that a hull design that operates acceptably at low speed may not operate acceptably at higher speeds.
Seaplanes compliment waterborne longitudinal stability with flight controls, i.e. elevator, with increased speed.
One type of longitudinal motion that may become pronounced at higher speeds is a phenomenon known as “porpoising,” which means that the watercraft tends to rhythmically pitch and translate vertically while travelling forward. This term “Porpoising” is the common term for watercraft that refers to the motion of the watercraft that is like the movement of a porpoise jumping out of the water.
Porpoising is a dynamic instability of any seaplane or high speed watercraft operating and may occur when the seaplane is moving across the water while on the step at high speed. It occurs when the angle between the hull and the water surface exceeds the upper or lower limit of the vehicles's pitch stability limits. Improper use of trim, propulsion and/or speed, may result in attaining too high or too low a pitch (trim angle) sets off a cyclic oscillation which steadily increases in amplitude unless the proper trim angle or pitch attitude is reestablished.
In most cases, porpoising is more likely to occur when the watercraft is at a higher speed than when the watercraft is at a lower speed.
However, as the seaplane's speed increases, the hull generates greater lift (which is a function of speed) and the wing generates additional lift.
This means that, at higher speeds, less of the seaplanes' hull contacts the water as it rises out of the water.
Another item more concerning in seaplane operations is the Roach (or Rooster Tail). The Roach is a high energy fountain or high arching spray generated by water flowing off of the seaplanes step. The dynamic pressure is converted into motion causing the water to fly into the air. The roach typically impact the aft end of seaplane (hulls and floats) causing a destabilizing nose down force and is a major contributor to porpoising. It could also impact other aircraft components such as the wing, propeller and tail causing instability and drag. Placing a hull inline (aft hull after the step) or spaced too close laterally can have unwanted impacts from the Roach upon the seaplane.
Seaplanes are generally divided into two categories, dedicated Flying boats or Floatplanes. Seaplanes originated without landing gear and only were able to operate on the water. Today, the term Seaplane, includes to amphibious aircraft. An amphibious aircraft is one that can operate on both land and water. Both Flying boats and Floatplanes are also known as amphibious aircraft.
Flying Boats typically have a single hull, a hydro-dynamically designed lower fuselage (hull). For longitudinal stability at stationary and low speeds, sponsons, wing floats, and occasionally multiple hulls/floats are used.
Amphibious Flying Boats typically have landing gear retracting into the hull or sponsons as the design dictates allowing amphibious operation.
Float Planes are normally land planes converted to operate on the water by the addition of floats. There have been floatplanes that are only seaplanes.
An amphibious Float Plane may have landing gear either inside or external to the floats that can be extended for land operations.
Float Planes and Flying Boats usually have a step. This insures the hydrodynamic lift is produced very close to the center-of-gravity allowing the seaplane to rotate for takeoff. Rotation allows a higher angle-of-attack thereby increasing the wing's lift allowing for a controlled takeoff.
The area aft of the step is referred to as the “afterbody”, that part of a seaplane hull or float aft of the main step and terminating at the sternpost, the aft end of the afterbody.
Incorporating the step with an afterbody or aft hull section inline, and not laterally spaced, can cause flow off of the step from striking the afterbody. This can result in longitudinal instability; Porpoising. All in-line or closely spaced lateral aft bodies reduce the trim angles of porpoise free operation.
An aft hull in line (after the step) with the front step(s), and hulls not laterally spaced from the step maybe impacted by the Roach, the flow off of the step. The roach striking the afterbody is the typical method of seaplane design reducing safe trim range of operation and resulting in the potential of Porpoising.
Conventional seaplane designs also tend to lack versatility in that while a particular design maybe suitable for use in specific environments, the same design may not be suitable for use in other environments. For example, a tri-hull boat configuration which may be quite efficient in smooth water conditions and at low speeds may not be suitable for rougher waters and at higher speeds.
Porpoising is induced from the interactions of the hulls used for longitudinal stability being inline or located too close laterally.
Single hull seaplanes, such as Flying Boats, utilize an inline stepped hull for longitudinal stability and sponsons or wing tip floats for lateral stability. Multi-hull seaplanes and most Float Planes use twin floats for longitudinal stability and lateral stability.
For static and low speed stability about the center of gravity, a minimum of three buoyant forces, are required for stability. These are generated by floats, hulls and/or sponsons. These maybe located either two forward, spaced apart for lateral stability and one aft balancing the forward buoyant forces for longitudinal stability. Conversely, the same can be accomplished with one buoyancy points forward and two buoyancy points aft. Additional floatation from additional hulls or floats may provide redundant stability.
A hull (or float) that includes a step and extends aft with a hull can be considered to have two buoyancy points for longitudinal stability in displacement mode; one forward and one aft of the center of gravity. Consequently, a twin hull float plane can be considered to have four buoyancy points; two laterally spaced in front of the cg and two laterally spaced behind the cg; in line with the front points.
A Flying boat with sponsons or wing tip floats is considered to have four points for stability. The main hull has a point forwards and aft, inline of the center of gravity with two points laterally spaced, from the sponsons or wingtip floats, for lateral stability. Additional hulls and or floats may be present providing additional stability forces.
Lateral stability at higher speeds is maintained by the hull, augmented by control surfaces as speed increases, allowing the seaplane to plane on the main hull, thereby having the wingtip floats or sponsons out of the water reducing hydrodynamic drag.
When Planing, the aft part of the hull, beyond the step are typically out of the water and not required for longitudinal stability since at higher speeds, the stability is provided by the hull plus elevator authority when available.
The aft hull used for static and low speed longitudinal is no longer required. Their location may be hazardous as they may impose porpoising from water or Roach impact. An alternate design eliminating this destabilizing Roach impact is possible.
Retractable gear is a design consideration and its incorporation into a boat hull or floats creates its own design challenges. Typically, the inline hulls, the area behind the steps are ustilized for the retractable landing gear.
Accordingly, there is a need for a multihull watercraft, configured with three or more hulls that is scalable, and which can provide a smooth, efficient ride over a range of speeds and water conditions to eliminate most porpoising modes for seaplanes and can also provide more efficient incorporation of the landing gear.